It is common to construct concrete duct structures that are designed to contain materials as they move from one location to another. The moving material is typically water. The ducts can be found in any environment, but usually they are placed underground to contain water movement.
Concrete ducts are put together in sections, with each section having a top, bottom and sides. Each section also has a male end (tongue or spigot) and a header female end (groove or socket) which then allows sections to be put together to create an extended duct. In order to properly contain fluids from escaping out of the duct and from preventing fluids and debris from entering the duct, a gasket is fitted to the male end to serve as a seal between the male and female ends upon mating of the sections.
It is imperative that the gasket be compressed a certain amount, typically defined by a percent of gasket compression. If the gasket is not compressed enough the duct connection could leak and if the gasket is compressed too much duct cracking could occur.
Theoretically, concrete ducts, which are measured by their inside dimension, can be any size. However, in actual practice there are certain standard interior sizes. The outer size, which is determined by the thickness of the respective walls, depends upon many factors, such as, depth underground, intended use, etc. Thus, it is necessary to be able to construct concrete ducts having varying dimensions and wall thicknesses.
Concrete ducts are formed by pouring (or shaking) concrete into a mold and then removing the mold when the concrete is dry (or compressed as in the case of near zero slump cast concrete). These molds must be constructed for each duct size and can be constructed on or near the point of installation. One major problem with current construction is manufacturing tolerances, which among other things, results in the gasket not being properly compressed.
Some prior art modular systems are large systems having a right and a left corner configuration. The primary reason for the different corner configurations is because the typical smallest size duct (interior) is 3 ft. by 2 ft. Thus, one corner has a leg of 3 ft. while the other corner has a leg of 2 ft. It is difficult to hold tight tolerance in larger structures, such as this for several reasons.
Some prior art modular systems with universal configuration for pallets and headers require two very different sets of mold equipment. For example, one set may cover a 4 ft.×2 ft. through 8 ft.×8 ft. product size with an 8 in. haunch size. The other set may cover a 6 ft.×3 ft. through 12 ft.×12 ft. product size with a 12 in. haunch size. There is an overlap in product size for the 6 ft., 7 ft. and 8 ft. span products when using the two different sets of equipment. When using the standard configuration set the 3 ft. span products were not addressed.
Because tolerances are hard to maintain, the industry has shied away from using rubber gasketed joints. Instead, they've used what's called a mastic joint which is a form of mortar joint. Workers would plaster mortar into the female end and then mate the male end of the next section. This mortar joint has been changed to a petroleum mastic product, but it remains difficult to obtain a water tight seal.